Methods of obtaining direct positive images using internal latent image type silver halide emulsions, which have not been pre-fogged by carrying out surface development either after or during the execution of a post-exposure fogging treatment, are well known.
The abovementioned internal latent image type silver halide photographic emulsion is a silver halide emulsion having photosensitive nuclei principally within the silver halide grains and with which the latent image formed by exposure to light is formed principally within the grains.
Various techniques for obtaining direct positive images are known. The principal techniques have been disclosed in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276 and 3,796,577, and British Patent Nos. 1,151,363, 1,150,553 and 1,011,062.
Comparatively high speed photographic materials can be obtained as direct positive type materials when utilizing these known methods.
Details of the formation and structure of the above mentioned positive image materials have been described by T. H. James in "The Theory of the Photographic Process", fourth edition (1977), chapter 7, pages 182-193, and in U.S. Pat. No. 3,761,276.
Fogging nuclei are produced selectively on the unexposed surfaces of the silver halide grains as a result of surface sensitivity reducing action brought about by the so-called internal latent image which is produced within the silver halide grains by the initial imagewise exposure; subsequently a photographic image (direct positive image) is formed in the unexposed parts of the grains by the execution of a normal so-called surface development process.
Known techniques for selectively forming the fogging nuclei as described above include, in general, "Light fogging methods" in which the whole photosensitive layer is subjected to a second exposure (as in British Patent No. 1,151,363) and "chemical fogging methods" in which a nucleating agent is used. The latter chemical fogging methods are discussed on pages 76-78 of Research Disclosure Vol. 151, No. 15162 (November, 1976).
A surface color developing process is carried out either after subjecting the internal latent image type silver halide photosensitive material to a fogging treatment or while executing such a treatment. The material is then subjected to bleaching and fixing processes (or a bleach-fix process) to form the direct positive color image. The material is further subjected to a normal water washing and/or stabilization process after the bleaching and fixing processes.
When forming a direct positive image using either the light fogging method or the chemical fogging method, the rate of development of the direct positive type material is slow and the processing time is longer than that of a normal negative type material. Thus, methods to shorten the processing time have been adopted, by raising the pH and/or the temperature of the development bath. However, there is a problem in that the minimum image density (Dmin) of the direct positive image obtained is generally increased at high pH. Further, the developing agent is liable to deteriorate as a result of aerial oxidation under conditions of high pH. Also, the pH is liable to be reduced by the absorption of carbon dioxide from the air. This results in a marked lowering of development activity.
N-Hydroxyalkyl substituted p-phenylenediamine derivatives are known developing agents which have a high level of development activity and which provide adequate maximum densities (Dmax). However, although a high maximum image density is obtained when direct positive images are formed using the abovementioned compounds, there is also a simultaneous increase in the minimum image density and thus there is a tendency towards softer gradation. It is therefore desirable to remedy these problems.
Accordingly, it is the objective of the present invention is to provide a method for forming direct positive color images having both a high maximum image density of 2.0 or more and a low minimum image density of 0.15 or less.
A further objective of the present invention is to provide a method for forming direct positive color images having excellent gradation and being suitable for practical applications.